1) Field of the Invention
The present invention relates to a metal insert which is at least partially embedded in a molded resin mass, and also relates to a method of surface treatment for toughening surfaces of such a metal insert.
2) Description of the Related Art
For example, as a metal insert which is partially embedded in a molded resin mass, a lead frame is well known to be used in the production of electronic components or electronic packages. In general, the lead frame comprises a central stage for supporting an electric element such as an integrated circuit (IC) chip, and a plurality of lead elements arranged around the central stage. In the production of electronic package, the IC chip is securely mounted on the central stage of the lead frame, and then is electrically connected to the inner ends of the lead elements by a well-known wire-bonding process. Thereafter, the central stage with the IC chip is enclosed together with the inner ends of the lead elements by a split mold, and then a suitable curable synthetic resin is introduced into the split mold. Thus, the IC chip is sealed in the molded resin mass in such a manner that the lead frame is partially embedded therein.
The lead frame may be formed from a thin Cu plate, Cu alloy plate, Ni-Fe alloy plate or the like by a mechanical punching process or a photoetching process, and is subjected to a surface treatment so as to improve an adhesion property with respect to the molded resin mass. For example, the surfaces of the lead frame are roughened by a mechanical pressing process or a chemical etching process, and thus the contact area between the lead frame and the molded resin mass is increased so that the lead frame exhibits an improved adhesion property to the molded resin mass. If the lead frame is not subjected to the rough-surface treatment, moisture can easily penetrate into the boundaries between the lead frame and the molded resin mass, so that cracks may occur in the molded resin mass due to evaporation of the penetrated moisture.
Nevertheless, when the lead frame is subjected to the rough-surface treatment by the mechanical pressing process, a residual strain is produced in the pressed lead frame, and is actualized as a warp in the lead frame under the thermal influence derived from the molding process. Also, in the chemical etching process for the rough-surface treatment, it is very difficult and troublesome to control a degree of roughness without reducing the strength of the lead frame.